As systems that perform near field communication, for example, IC (Integrated Circuit) systems are widely known. In an IC card system, a reader/writer forms what is called an RF (Radio Frequency) field (magnetic field) by generating an electromagnetic wave. Then, when an IC card comes close to the reader/writer, the IC card receives supply of electric power by electromagnetic induction and performs data transmission with the reader/writer.
In the IC card system, if a plurality of IC cards come close to one reader/writer, the reader/writer has to identify each of the plurality of IC cards, specify a counterparty of communication, and perform communication.
As a method for identifying a plurality of IC cards, a method in which IDs as unique identification numbers are assigned to the IC cards and the IDs are reported from the IC cards to a reader/writer has been proposed. If unique IDs are assigned to the IC cards as described above, IDs do not coincide among IC cards.
Furthermore, a method in which a random number is generated and the random number is temporarily used as an own ID in an IC card has been proposed. When random numbers are used as IDs, it could possibly occur that the same random number is used as an ID in a plurality of IC cards. In this case, if a reader/writer sends data to the ID, the plurality of IC cards respond simultaneously to cause interference (collision), so that the reader/writer is not allowed to normally obtain a response from the IC cards.
A proposal has been made to accurately identify each of a plurality of counterparties of communications and prevent receiving responses simultaneously from two or more counterparties of communications while preventing such a situation (e.g., refer to Patent Document 1).    [Patent Document 1] Japanese Patent No. 3695464